Ceres, Vesta and the Dawn Mission

by Paul Gilster on June 23, 2007

With launch of the Dawn mission to Ceres and Vesta coming up on July 7, NASA has announced a news conference for next Tuesday, the 26th, to discuss details of the four year journey to the asteroids. Held at NASA headquarters, the event is due to be streamed on the agency’s homepage. The Hubble Space Telescope, meanwhile, has provided these images, shown below as a montage, of the two target asteroids. The debris of the asteroid belt, which may house 100,000 or more asteroids as large as ten kilometers across, provides an idea of the kind of materials available for planet-building some 4.6 billion years ago.

For those who follow robotic missions with fascination for the rapid strides in technology they represent, consider that Dawn is the first mission sent to orbit two different targets. Vesta will be the first, in 2011, with Ceres following in 2015. The Vesta image (on the right) shows the asteroid’s southern hemisphere, which is dominated by an impact crater so large that the distance across it is almost equal to the asteroid’s diameter. About the size of Arizona, Vesta produced fifty smaller asteroids from the impact that are often referred to as ‘vestoids.’

Image: These Hubble Space Telescope images of Ceres and Vesta show two of the most massive asteroids in the asteroid belt, a region between Mars and Jupiter. The images are helping astronomers plan for the Dawn spacecraft’s tour of these hefty asteroids. Credit: For Ceres, NASA, ESA, and J. Parker (Southwest Research Institute). For Vesta, NASA, ESA, and L. McFadden (University of Maryland).

Ceres, seen on the left, shows the presence of dark and bright regions probably related to topographic features. The small world is thought to hold thirty to forty percent of the mass in the asteroid belt, with water possibly occurring beneath its surface. Or maybe we should call this a ‘dwarf planet,’ as planet-definers like the IAU prefer to do. Ceres is round and thus planet-like, but it does not sweep debris out of its orbit. However we describe it, this first asteroid to be discovered (in 1801) should tell us much about asteroid structure, and the comparison with New Horizons data from Pluto may help us decide whether the ‘dwarf planet’ category is sufficient to cover both of these small worlds.

I have seen this claim before and it is wrong. Hyperbolic orbits are orbits too, so there have been dozens of other missions that orbit two targets (think of gravity assist). Dawn will be the first mission to orbit two asteroids.

This seems a stretch to me, Ed. Surely you wouldn’t argue that Voyager II is ‘orbiting’ Jupiter, not in the commonly accepted usage of the term? Yes, there are such things as hyperbolic orbits, but I think the meaning here is clear. For those interested, here’s Wikipedia on hyperbolic trajectories:

I wonder how much of the asteroid belt Ceres will have gobbled up in a billion years or so – and how large it will ultimately become. Maybe even large enough to have it’s own biosphere once the expanding sun moves the habitable zone out to Ceres’ orbit. Or is there enough mass in the asteroid belt for Ceres to ever become a true planet?

The total mass of the asteroid belt is very small, comparable to that of the 2nd tier of moons of the outer planets (roughly similar to the mass of Uranus’s moon Oberon). And the majority of that mass is already concentrated in the few largest asteroids. Ceres is unlikely to get much larger than it is today.

To put it into prespective, Ceres is roughly the size of Texas (surface area). It’s mass is nearly 50% of the total mass of the asteriod belt. Vesta’s surface area is only about the size of Connecticut. So, clearly, if we ever wanted to colonize an asteriod, Ceres would be the first candidate.

Best use for Ceres’ mass is to convert it into a massive space habitat, if we’re talking about a super-industrialised solar system. In the near term it’s a fascinating target for “Dawn”.

Theoretically diamond fibre would have a strength of 2 TeraPascals, thus allowing a 1-gee habitat ring 13,000 km across with a safety factor of 2. All the carbon in Venus’s atmosphere could make a few dozen, while Ceres’ carbon (~3%) would mass about 30% of that.

Cameron Park (SPX) Jul 02, 2007 – As the “Dawn” mission — the first attempt to explore truly large asteroids — moves, somewhat uncertainly, toward its hoped-for July 7 launch, we’re still finding out new details about its mission. The mission’s principal Investigator, C.T. Russell, outlined two of them to SpaceDaily in a recent interview. (1) It’s been assumed for some time that Dawn — in addition to orbiting both of:

I have wondered for some time why the larger asteroids have received comparitively little attention in the way of robotic probes in the past. I suppose it is because they are perhaps regarded as being less spectactular than their neighbours, Mars and Jupiter.

From a mixed commercial, exploratory and technology demonstrator point of view, however, dwarf planets like Ceres and Vesta possibly offer easier access than Mars due to low gravity, a good anchor for tools or a robotic station, also due to the gravity, potential protection from solar radiation by tunnelling into them, just enough solar radiation that solar energy would still work (albeit at a much reduced rate) and a good place to begin trials of automated robotic asteroid mining and manufacturing. Such automated trials would also be an excellent way to build a really large observatory in microgravity, I think.

In other words, in my own somewhat subjective view, the larger asteroids would be ideal starting points for scientific discovery and commercial exploitation of the solar system.

Abstract: We study the physical characteristics (shape, dimensions, spin axis direction, albedo maps, mineralogy) of the dwarf-planet Ceres based on high-angular resolution near-infrared observations. We analyze adaptive optics J/H/K imaging observations of Ceres performed at Keck II Observatory in September 2002 with an equivalent spatial resolution of ~50 km. The spectral behavior of the main geological features present on Ceres is compared with laboratory samples.

Ceres’ shape can be described by an oblate spheroid (a = b = 479.7 +/- 2.3 km, c = 444.4 +/- 2.1 km) with EQJ2000.0 spin vector coordinates RA = 288 +/- 5 deg. and DEC = +66 +/- 5 deg. Ceres sidereal period is measured to be 9.0741 +/- 0.0001 h. We image surface features with diameters in the 50-180 km range and an albedo contrast of ~6% with respect to the average Ceres albedo. The spectral behavior of the brightest regions on Ceres is consistent with phyllosilicates and carbonate compounds. Darker isolated regions could be related to the presence of frost.

“Best use for Ceres’ mass is to convert it into a massive space habitat, if we’re talking about a super-industrialised solar system.”

In that case, you’ve got to accept it’s also the best use for Mercury, Venus, the Moon, Earth, Mars, Jupiter, it’s Moons, etc…

I know, let’s all live in a ring orbiting the Sun, as it provides the best population density. No matter that it requires destroying the Solar system.

No need to do that. I know people will probably accuse me of Planetary Chauvinism, but I don’t see any reason to dismantle the main sights of the Solar system for space habitats.

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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